(1) TECHNICAL FIELD
This invention relates to a thermal energy scavenger or a thermal energy converting assembly of the type for converting heat energy into mechanical energy and, more specifically, to such an assembly utilizing a plurality of temperature sensitive elements made of a material which exhibits shape memory due to a thermoelastic martensitic phase transformations whereby less energy is required to strain the elements in a cold condition than the energy returned when the elements become unstrained as they are heated to a higher temperature.
(2) DESCRIPTION OF THE PRIOR ART
During recent years various materials comprising metal alloys have been developed which have a shape memory characteristic based upon thermoelastic, martensitic phase transformations which are stress or stain dependent. Basically, such alloys exhibit a stable shape in a phase above a given transformation temperature and experience a transformation to a martensitic phase at a temperature below the transformation temperature The alloys have a much lower effective modulus in the martensitic phase below the transition temperature thereby requiring a relatively small amount of work in the form of stress and stain for straining the alloy when at the lower temperature. Further, the alloy provides much more work as it unstrains and returns to its original shape when it reaches a higher temperature above the transition temperature. Examples of alloys which have this shape memory characteristics are nickel-titanium; copper-aluminum-nickel; copper-zinc; iron-platinum and gold cadmium.
However, it should be noted that in order for the temperature sensitive elements made of the alloys as described above to have the shape memory characteristic, each element must be given a shape memory anneal. This heat treatment establishes the memory shape that the element will assume when heated above the phase transformation temperature. For example, the nickel titanium alloy exhibits excellent shape memory response when heat treated (memory annealed) at between 300.degree. to 600.degree. C. However, the unannealed, as drawn or as rolled state of the nickel titanium alloy, after careful repeated cold working, has yielded material with tensile yield strengths in excess of 200,000 psi with an elastic modulus of about 4.times.10.sup.6 psi while still in the martensitic phase. Such material makes excellent springs, and in fact can store significantly more energy than steel because of the eight fold reduction in the modulus of elasticity with no sacrifice in yield strength.
A discussion of shape memory characteristics in a number of alloys is set forth in the Journal of Material Science; 1974, Volume 9, pages 15-21 by authors L. Delaey, R. V. Krishnan and H. Tas. Further discussions are set forth in Metallurgical Transactions; 1975, Volume 6A, page 29 by H. C. Tong and C. M. Wayman.
Further descriptions of materials having the shape memory characteristics are set forth in U.S. Pat. No. 3,174,851 granted Mar. 23, 1965 to William J. Buehler and Raymond C. Wiley and U.S. Pat. No. 3,558,369 granted to F. E. Wang and William J. Buehler on Jan. 26, 1971.
There have been efforts to utilize these materials, which have shape memory characteristics, in thermal energy converting assemblies and such assemblies have proven that the material may be so utilized. More specifically, there are various assemblies known in the prior art which utilize a thermal energy transfer between a fluid medium and a material which responds to thermal energy transfer to produce motion and work. Typically, the assemblies alternately subject the material to hot and cold fluids such as hot and cold water. In one such assembly, tubes of the temperature responsive material have alternate flows of hot and cold water through the tubes for resulting in an overworking of that portion of the tube forming the interface between the hot and cold as the tubes are heated from one end to the other. An example of this assembly is shown in U.S. Pat. No. 3,937,019 granted Feb. 10, 1976 to E. Renner. U.S. Pat. No. 4,041,706 granted Aug. 16, 1971 to F. I. White discloses a plurality of wire elements of thermally reactive material working together in a bundle with a rotating valve for alternately subjecting the wires to hot and cold liquid.
As this technology has developed, newer assemblies have been introduced into the prior art and have been directed at solving problems associated with the straining and unstraining of the temperature sensitive elements. For example, and more recently, U.S. Pat. No. 4,197,708 granted Apr. 15, 1980 to Milton, Jr. et al discloses a thermal energy scavenger which incorporates alternating stress limiters for limiting the stress applied to the temperature sensitive elements below a safe level in order to avoid plastically deforming the elements. This assembly includes a reaction mechanism which reacts with the elements for applying a stress to the elements to strain the elements during a first phase and for responding to the unstraining of the elements during a second phase. A carriage assembly supports the first and second ends of the elements for allowing the elements to be placed in tension while reacting with the reaction mechanism. The stress limiters are disposed between only one end of each of the elements and the carriage assembly for limiting the strain of the elements during the phases as stress is transmitted between the elements and the reaction mechanism through the stress limiters. The stress limiters as disclosed in the Milton Jr. et al patent includes a plurality of coiled springs which react between associated wire elements and the carriage assembly to limit the stress applied to the individual temperature sensitive elements below a predetermined level.
Unfortunately, a practical application of the above described stress limiters and as set forth in the '708 patent to Milton et al, could not be achieved for each of the very closely spaced individual wires in the bundle because of the use of the helical stress limiting springs.
Other problems relating to the transfer associated with the cyclic heating and cooling of temperature sensitive elements have been addressed in the prior art. For example U.S. Pat. No. 4,306,415 issued on Dec. 22, 1981 to Hochstein et al discloses a thermal energy scavenger employing a plurality of modular bundles of temperature sensitive elements made of nickel titanium alloy wire elements. With this design, it is important that the entire bundles of wires be quickly and efficiently cycled between the relatively hot and cold fluid environments so as to take advantage of the attendant phase transformation in the temperature sensitive elements while straining and unstraining the wire elements. To this end, the Hochstein et al '415 patent discloses a fluid flow control assembly calculated to subject the entire effective working length of the wire bundles across the width of the bundles to a flow of fluid flowing generally perpendicular to the length of and about the elements so that all portions of the length of each element are simultaneously subjected to the fluid. However, it is a practical reality that not all of the wires in the bundle will be subjected to the fluid flow simultaneously. Accordingly, the first wire is subjected to the fluid will undergo a phase transformation between austensite and martensite, and vice versa, before the phenomena can take place in the last wire subjected to the perpendicular fluid flow. When the entire modular bundle of wires is strained before all the wires in the bundle have been properly cooled to below the phase transformation temperature, this can result in the cold working of some of the lagging wires. Repeated cold working of the wires robs them of the ability to remember their shape after straining. The power output of the thermal energy scavenger is thereby reduced and the wires can ultimately break.
There have been other problems in the prior art assemblies associated with anchoring or fixedly supporting the temperature sensitive elements at their ends. Typically, the wire elements as employed in the prior art have been anchored by burying the material within a structure consisting of a clamp, screw or similar mechanism. See for example the Milton, Jr. et al '708 and the Hochstein et al '415 patents. Unfortunately, these anchoring systems have been relatively costly, difficult to produce, and have resulted in wire breakage.
The subject invention overcomes the deficiencies in the prior art in a more efficient, relatively simple thermal energy scavenger assembly which is capable of producing a higher energy density than those presently known in the prior art.